Fix anthropicApi visibility in AnthropicChatModel

[Hyune-c]

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I updated the AnthropicChatModel by changing the anthropicApi field from public to private.

• Ensures consistency since all other major variables are private
• Aligns with how other AI provider models handle their APIs
• Verified that anthropicApi is only used internally, avoiding external issues

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[markpollack]

thanks, merged in e123721